Not applicable.
Not applicable.
Dense solid electrolyte membranes formed from certain classes of multicomponent metallic oxides transport oxygen ions at elevated temperatures upon application of an electric potential gradient across such dense membrane. These devices are referred to as electrically driven solid electrolyte oxygen separation devices. Dense solid electrolyte membranes, which do not possess connected through porosity, transport oxygen ions an upon application of an electrical potential gradient across the dense solid electrolyte membrane.
Each electrochemical cell comprises a dense solid electrolyte membrane formed from an oxygen ion conducting multicomponent metallic oxide, an anode and a cathode. Two or more of such electrochemical cells are connected in series by interconnects which are formed from electron conducting multicomponent metallic oxides. An interconnect is defined as an element which connects an anode and cathode of immediately adjacent electrochemical cells to establish an electrical connection in series between such adjacent electrochemical cells.
The above electrochemical cells can be constructed in tubular, flat plate and honeycomb configurations. The flat plate configuration is preferred for several reasons since it allows for multiplication by connecting several electrochemical cells comprising said solid electrolyte membranes in a stack. In such a stack, a plurality of electrochemical cells comprising the dense solid electrolyte membranes are combined (or stacked) to operate in electrical series. This in turn increases the efficiency of the device. The flat plate design is also favored for ease of assembly and compact dimensions.
The stack may optionally include a support member and anode and cathode seals. The stack of these electrochemical cells may be placed between an anode and a cathode connection on respective end plates and may be housed in a shell providing for manifolds, heating etc.
Representative structures are disclosed in U.S. Pat. Nos. 5,868,918, and 5,570,279, both assigned to Air Products and Chemicals, Inc., and U.S. Pat. Nos. 4,885,142; 5,186,806; 5,298,138 or European Patents Nos. 0 682 379 and 0 983 786.
The interconnects of these subject devices fulfill several roles. The interconnect (1) provides for separation of gas passages between anode and cathode sides of adjacent electrolyte plates, (2) provides the channels by which feed and product gas streams are manifolded, (3) acts as an electronic conductor to connect the solid electrochemical cells in series, (4) prevents back diffusion of oxygen from the product stream to the feed stream, and (5) in many cases due to the relative thickness of the components, the interconnect provides additional mechanical support to the stack.
Interconnects are formed from electrically conductive materials which have low oxygen ionic conductivity under operating conditions, typically an oxygen ion conductivity of less than 10xe2x88x922 S/cm. Interconnects are formed from compositions which conduct electrons under operating conditions, and which have a low oxygen ion conductivity under operating conditions. Such interconnects must be sufficiently compatible with other device materials so that the interconnect should not adversely react with other components to form products which negatively impact device performance or lifetime. The interconnects should possess a coefficient of thermal expansion that matches other device materials, and have sufficient mechanical stability to withstand the prevailing pressure difference within each electrochemical cell. The interconnect material should be stable at the conditions prevailing at the anode and cathode side of the solid electrolyte membrane. The interconnect should be of sufficient strength to mechanically stabilize the stack.
Further, the interconnect material should be formed from a composition of matter which will not deform or distort upon either assembly or use of the device. When the above material demands are combined, the number of candidate materials for making the interconnects is severely limited.
Stoichiometric lanthanum strontium manganite represents a commonly used interconnect composition. U.S. Pat. No. 5,750,279 discloses a series planar design for solid electrolyte oxygen pumps. This patent lists a number of candidate stoichiometric compositions for interconnects including lanthanum strontium manganite, lanthanum strontium chromite, lanthanum calcium manganite, and lanthanum calcium chromite. (see also, U.S. Pat. No. 5,868,918).
The mechanical properties of stoichiometric lanthanum strontium manganite interconnects (LSM-interconnects) are not completely satisfactory. For example, sintered-interconnects formed from stoichiometric LSM may display room temperature deformation properties at moderate stress.
The prior art stoichiometric LSM-interconnects exhibit low values for dynamic Young""s modulus and fracture strength. The presence of microcracking or other phenomena relating to low modulus, low strength, and interconnect deformability may limit the long term mechanical performance of the apparatus.
Those skilled in the art are searching for a mechanically stable and electronically conductive, and economically viable interconnect for use in electrically driven solid electrolyte oxygen separation devices.
This object is solved and the above deficiencies and other disadvantages of the prior art are overcome by an interconnect for an electrically driven solid electrolyte oxygen separation device comprising a composition of matter represented by the general formula:
LnxCaxxe2x80x2Axxe2x80x3MnyByxe2x80x2O3xe2x88x92xcex4
wherein Ln is selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; A is selected from the group consisting of Sr, Ba, and Y; B is selected the group consisting of Cu, Co, Cr, Fe, Ni, Zn, Nb, Zr, V, Ta, Ti, Al, Mg, and Ga; 0.1xe2x89xa6xxe2x89xa60.9; 0.1xe2x89xa6xxe2x80x2xe2x89xa60.9; 0xe2x89xa6xxe2x80x3xe2x89xa60.5; 0.5 less than yxe2x89xa61.2;and 0xe2x89xa6yxe2x80x2xe2x89xa60.5; provided that x+xxe2x80x2+xxe2x80x3=1and 1.2 greater than y+yxe2x80x2 greater than 1.0, wherein xcex4 is a number which renders the composition of matter charge neutral.
Another embodiment of the present invention relates to an electrochemical device comprising such interconnect. More in detail, the invention relates to an electrochemical solid-state device comprising at least two electrochemical cells which are electrically connected in series by one or more interconnects wherein at least one interconnect comprises a composition of matter represented by the general formula:
LnxCaxxe2x80x2Axxe2x80x3MnyByxe2x80x2O3xe2x88x92xcex4
wherein Ln, A, B, xcex4, x, xxe2x80x2, xxe2x80x3, y, and yxe2x80x2 are as defined above.